Leonard Susskind - Why is Quantum Gravity Key?

Lenard the effort to unify quantum mechanics the great discoveries about the microscopic world the early 20th century and the general relativity the macroscopic world the explanations that quest a unified who is the holy grail of physics you've come at it through a long history with string theory I'd love to hear how how it happened and where we are today good how it happened was like many discoveries somewhat accidental you discover something in a context which turns out not to be the natural context for studying it the string theory originated as a theory of protons and neutrons and massan's these are objects about a hundred thousand times smaller than a atom very big by physicists standards enormous in fact great big fat marshmallows protons neutrons massan's they are string like you can stretch them out you can they're almost like Turkish taffy you pull on them and they stretch out it was an effort to describe them that the mathematics and the constructions that went into string theory were first invented by myself by Nambu by other people and we described this mathematics that was supposed to be describing protons and neutrons and it did a pretty good job of that incidentally it did a pretty good job of that but nature repeats itself apparently and on scales a billion billion times smaller that same mathematics seems capable of describing physics at the Planck scale it seems capable perhaps of describing the physics of quantum gravity a billion billion times smaller than it was originally set out to describe so in a sense it was an accidental discovery intended to be describing something quite different than quantum gravity it was incidentally other people who observed that if you took the same mathematics and shrunk it and sped it up incidentally by a billion billion times it was capable of describing gravity now these strings have an oscillation nature to them and simply it's almost like a oscillating vibrating violin string absolutely well maybe more of a rubber band uh-huh it's not stretched out and long it's more like a little closed rubber band and if the rubber band didn't have friction you could imagine it vibrating and vibrating and vibrating forever that's the character of these strings but remember they are quantitatively very different than rubber bands that billion billion billion times smaller than rubber bands okay and it's interesting to ask how strong they are I once calculated that they are about strong enough that at the surface of the earth if you could connect one end to a two 10 to the 40th Mack trucks you could lift up the 10 to the 40 of the Mack trucks with them and that's what what one string that's why they're so small because they're so difficult to pull apart so they are very thin very small and very very strong now out of these out of the vibrations of these strings all of the particles forces that we know can be derived or can fall out I mean that's the hope you said that's the hope that by no means has that been done sure what string theory is at the present time is it's a mathematical structure which looks more like the universe or more like the universe of elementary particles than any other theory which contains gravity all other theories in fact quantum field theory not only don't contain gravity they can't contain gravity gravity is inconsistent with them string theory has the property that automatically contains gravity it can't avoid gravity it also contains particles it contains particles that look roughly like quarks particles that look like photons particles that look like gravitons but it has never been put together in a form which is so close to natural reality that we're certain that it's correct it has never been put together in that form it's a mathematical structure and for me one of the things about that mathematical structure is it's so mathematically precise that we can try out ideas in it ideas like for example how black holes work ideas for example like the holographic principle we can try them out in a very very precise mathematical context and see whether they make mathematical sense that probably has been the most important aspect of string theory up till now trying out and testing the consistency of ideas about how quantum mechanics fits together with gravity at the present time it is quite far from describing nature with any great precision but the hope is because we have a few dozen a couple dozen parameters free parameters so-called the constants of physics which look very random in the standard model of particle physics absolutely there is one in in in in string theory that if you one of many and that can that can do all of them but as time went on that one became the selected in five and now we're up to huge numbers numbers of ways to choose that one but still it's one that expresses all and those huge numbers probably account for the reason that we've had so much difficulty in finding one precise one that describes our world this landscape of possibilities is an enormous haystack and we're looking for a tiny tiny needle in an enormous haystack so it will probably be exceedingly difficult not totally impossible to find our one precise point in this landscape far far more than looking for one particle in the whole universe far more far more so partly we now understand why it is so difficult nevertheless we have found constructions or examples of it which look a good deal like like the real world the place where we have not made a lot of progress is unifying string theory with cosmology to some extent we have and to some extent it still eludes us my own feeling is that we are really missing some very very big pieces that string theory is not the end of the story that string theory is a point along a trajectory where the trajectory goes on and on in many different ways that we know they're not yet on the story there are two ways you can understand what you just said either it is a way of thinking which will lead us to other ways of thinking that's or it is it is something that explains say the standard model but something else has to sit beneath it to explain it may both may be true I think both may be true there may be in string theory a construction which looks very much like the standard model may be enough to say that's it that could be I think it's probably true my own feeling is that is probably true and that we will eventually get fairly close to it but the question then is is is that the end of a story does that tell us how the universe was born why the universe was born does it answer the really hard questions does it answer the question about why quantum mechanics does it answer all of puzzling puzzling questions about quantum mechanics and they are puzzling and the answer is no the answer is no well you're just getting started I have confidence but that's the point we are just getting started and anybody who thinks that were near the end of the story I think is misguided I think we can expect a lot of surprises in the future and who is going to be most surprised it's the people who have the strongest opinions about things and that means string theorists there are going to be big surprises the question is whether the doorways that are opened by string theory lead anywhere and yes I think they lead somewheres do is that doorway that we're just opening now the last doorway not likely

Channel: Closer To Truth

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Keywords: closer to truth, robert lawrence kuhn, Leonard Susskind, quantum mechanics, black holes, quantum gravity, string theory, quantum physics, Quantum theory, general relativity, theory of general relativity, structure of the universe, general relativity vs quantum mechanics, physics, education, Theoretical Physics, Stanford Institute for Theoretical Physics, leonard susskind quantum mechanics, quantum mechanics vs general relativity, quantum mechanics forbids this, quantum

Id: DSPDw1mJAiE

Length: 9min 18sec (558 seconds)

Published: Sun Jun 14 2020